Catalytic gas heater screen system

ABSTRACT

A system for oxidation of carbon monoxide produced from gas log sets used in a vent free mode in a wood burning fireplace or fireplace insert. In the system a vent chamber is placed in the top of the combustion chamber. The vent chamber has three sides, a ceiling and a floor the front of the chamber being open and venting outside of the firebox. The vent chamber is positioned in the top of the fireplace combustion chamber. The floor of the vent chamber has a hole cut into it and a low shape profile, low pressure drop catalytic screen placed in the hole so that the exhaust can pass through the catalyst into the chamber and then vent to the room. A curtain may be placed below the chamber vent and in front of the catalyst to act as a baffle or dam to restrict the flow of rising hot exhaust and redirecting it through the catalyst.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] 2. Related Art

[0003] Vent free gas burning appliances are growing in popularity anduse. The high thermal efficiency, low cost, ease of installation and thecleanliness of these heating devices make them attractive to consumers.The downside of the use of vent free products is that the products ofcombustion such as carbon dioxide, water and sulfur oxides can bedetriments to indoor air quality and oxygen depletion and carbonmonoxide emissions are dangerous. Several manufacturers supply vent freegas fireplace products with oxygen depletion sensors and oxidationcatalysts for carbon monoxide destruction. In order to make the catalystfunctional the air must be directed through the catalyst structure. Thisis accomplished by enclosing the combustion chamber with a glass orglass ceramic front surface and placing the catalyst so that the normaldrafting process produces flow through the catalyst. This arrangementproduces exhaust with essentially no carbon monoxide.

[0004] Vent-free gas fireplaces, stoves and vent-free log sets have beenavailable for a number of years. In the past few years numerouscatalytic vent-free versions have been introduced. The advantages ofcatalytic vent-free versions lie in the improved safety afforded by thepresence of the catalyst to reduce carbon monoxide emissions. Thecatalyst is usually supported on large cell honeycomb structure that maybe either ceramic or metal in composition. Because this structurecreates a flow resistance, however small, the catalytic vent-freeappliance requires a closed combustion chamber in order to force theflue gases through the catalysts. In order to maintain the aesthetics ofthe appliance, a high temperature glass door is utilized.

[0005] Many more gas fireplaces are in service, which use the so-calledgas log set. These sets are made from a burner arrangement, whichproduces attractive flickering flames over artificial ceramic logs. Manyof these log sets have been designed to be placed in a wood burningfireplace and to have the exhaust removed by a chimney. Still othershave been designed to be placed in a fireplace insert or a wood-burningfireplace in which the damper is sealed to produce a vent freearrangement. These arrangements give some of the advantages of the ventfree products are not known for use with a catalyst to destroy carbonmonoxide due to the open nature of the combustion chamber.

[0006] The present invention employs a very low pressure drop catalystcoupled with a fully or partially catalyzed screen to eliminate the needfor the expensive high temperature glass and yield a more conventionalfireplace look.

SUMMARY OF THE INVENTION

[0007] The present invention is a system to provide for oxidation ofcarbon monoxide produced from gas log sets used in a vent free mode in awood burning fireplace or fireplace insert. The system comprises a ventchamber positioned at the top of a combustion chamber. The vent chamberhas three sides, a top and a floor. The front of the vent chamber isopen and vents outside of the combustion chamber. The floor of the ventchamber which is contiguous with the combustion contains an opening. Alow shape profile, low pressure drop catalytic structure is placed inthe opening in a frame so that a portion of the exhaust passes throughthe catalytic structure into the vent chamber and then vents to theroom. A curtain may be placed below the vent of the vent chamber and infront of the catalytic structure to act as a baffle or dam to restrictthe flow of rising hot exhaust and redirecting it through the catalyticstructure.

[0008] In a preferred embodiment the invention is a system to providefor oxidation of carbon monoxide produced from gas log sets used in avent free mode in a wood burning fireplace or fireplace insertcomprising a vent chamber positioned at the top of a combustion chamber,said vent chamber having three sides, a top, a floor having an openingtherein for gaseous communication with said combustion chamber, an openfront to vent outside of the combustion chamber having a mouth, acatalytic structure is positioned in the opening in a frame so that aportion of exhaust from burning within the combustion chamber passesthrough the catalytic structure into the vent chamber.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009]FIG. 1 is a schematic front elevational view representation of afire box employing one embodiment of the present invention.

[0010]FIG. 2 is a schematic side view elevational view of an embodimenthaving a heat defector.

[0011]FIG. 3 is a top view of a filter in a frame.

DETAILED DESCRIPTION

[0012] The vent chamber height ranges from 0.5 to 6 inches. In oneembodiment the vent chamber is formed as a portion of a fireplaceinsert.

[0013] The curtain may be made of stainless steel that has beenoxidized, coated with a catalytic coating and mounted hanging from thefront of the chamber floor into the combustion chamber. The curtain sizemay be height of the fireplace open face. The curtain may be a knittedor woven metal screen typically used as a fireplace curtain and may hangon a sliding rod and can be pulled aside to expose the flame. Thecurtain may also be coated with a catalytic coating. The curtain mayalso be made of non porous metal sheet or foil.

[0014] Preferably the catalytic structure comprises a screen. The screenis catalyzed either over a portion or over its full length in order toreduce the level of carbon monoxide in the gas that escapes through thescreen. The screen may consist of any type of structure, be it woven,knitted, or otherwise, which preferably has more than 30% open area, towhich the catalytic material can be fixed. More preferably the screenshave more than 6 meshes per square inch and less than 600 meshes persquare inch and open area of 35 to 75%. The screens, where more than onescreen is employed, may be the same or of different mesh and open area.

[0015] The catalytic structure is preferably one or more screens made ofstainless steel which have been oxidized, coated with a catalyticcoating and mounted in the hole in the chamber floor. The catalystcoating is preferably a high surface area ceramic chosen from aluminumoxide, cerium oxide, titanium oxide, zirconium oxide or mixtures thereofand coated with catalytic components comprising a noble metal,preferably platinum, palladium, cobalt or mixtures thereof.

[0016] Referring to FIG. 1 a schematic representation of a conventionalfirebox 10 having an interior vent 14 is depicted. A gas log set 12 isfrequently used to provide heat. This conventional arrangement, howevermust rely on precise and careful adjustment of the flame to keep carbonmonoxide levels low. In FIG. 2 this conventional setup has been modifiedin accordance with the present invention with a filter 18 seated on anopen or perorated structure 28 which allows passage of the combustiongases through the filter and into the vent chamber 24. An extension 16may be added to the original vent if necessary to direct the filteredgas to the vent chamber for distribution. This feature is particularlyrelated to modified conventional fireboxes. Preferably a curtain or dam26 extends from the filter bearing structure to aid in channeling thecombustion gases thru the filter rather than out of the open face 30 ofthe firebox.

[0017] In FIG. 3 a preferred form of filter comprising a frame 20 whichis adapted to fit over or into opening or perorations in the structure28 which contains a low pressure drop catalyst such as a metal wire meshcoated with and oxidation catalyst.

EXAMPLE 1 Control

[0018] A gas log set rated at 32,000 BTU/hr was connected to propanefuel tank and placed into a fireplace insert as shown in FIG. 1. Thefire was lit and the emissions were monitored for oxygen, carbon dioxideand carbon monoxide. Oxygen and carbon dioxide levels were determined byan Illinois instrument model #3530 and carbon monoxide was determinedusing a TECO model #48 non dispersive infrared analyzer. The exhaust wassampled at the outlet of the overhanging hood just as the gases exhaustfrom the top of the firebox as shown in FIG. 1b. The sample probe wasconstructed of stainless steel 0.75 inch diameter pipe which, was 36inches long and held eight one eighth inches diameter tubes each 3inches long and spaced 4 inches apart. The sample was extracted by adiaphragm pump and forced into a gas dryer and then into the analyticalinstruments for gas analysis. The carbon monoxide values were correctedto moisture and oxygen free values by the following calculation:

Air Free CO═CO measured×13.76/Carbon Dioxide measured

[0019] In the first experiment the measured CO is as shown in Table 1.TABLE 1 Baseline Carbon Monoxide Levels. Oxygen and Moisture free Time,minutes CO ppm CO₂ % Calculated CO ppm 0 0.5 0.1 1 16.6 0.8 276.9 2 15.81.0 210.5 3 15.8 1.0 210.5

EXAMPLE 2

[0020] An experiment was carried out as in Example 1 except that achamber was produced by inserting a frame which was constructed to holdone layer of stainless steel wire mesh with 30 meshes per inch. Thescreen had been coated with 1% Pt on alumina wash coat which had beenmilled to less than 2 micron diameter and applied by spray coating. Thecatalyst coating was treated at 500° C. for one hour and the coatedscreen was fitted to the frame. The frame was placed in the top of thefireplace insert to allow a 1.5 inch space between screen and ceiling.The carbon monoxide reading was 8.6 ppm and carbon dioxide was 1.5%.When these data were corrected to oxygen and water free conditions thecarbon monoxide was 78.8 ppm indicating a 62% destruction of carbonmonoxide.

EXAMPLE 3

[0021] The same experiment was performed as in Example 2, except that 2layers of 30 mesh, catalyzed screen was installed. The carbon monoxidewas measured at 2.4 ppm 1.6% CO₂. The calculated oxygen free and waterfree carbon monoxide level is calculated at 20.6 ppm indicating a 90%destruction of carbon monoxide.

EXAMPLE 4

[0022] The same experiment as in Example 3 was performed except that astainless steel foil 1.5 inches wide was placed below and in front ofthe catalyzed screens to force more of the exhaust to go through thescreen as shown in FIG. 2. The measured carbon monoxide was 1.3 ppm andthe CO₂ was 1.7%. Calculated oxygen and moisture free carbon monoxidewas 10.5 ppm. This indicates a carbon monoxide destruction efficiency of95%.

EXAMPLE 5

[0023] The same experiment was performed as in Example 2, except that 2layers of 18 mesh, catalyzed screen was installed. The carbon monoxidewas measured at 3.9 ppm and carbon dioxide was 1.5%. The calculatedoxygen free and water free carbon monoxide level was 35.8 ppm indicatingan 82.9% destruction of carbon monoxide.

EXAMPLE 6

[0024] The same experiment was performed as in Example 5, except that 3layers of 18 mesh, catalyzed screen was installed. The carbon monoxidewas measured at 2.5 ppm and carbon dioxide was 1.4%. The calculatedoxygen free and water free carbon monoxide level was 24.6 ppm indicatingan 88.3% destruction of carbon monoxide.

EXAMPLE 7

[0025] The same experiment was performed as in Example 5, except thatthe space between the catalyzed screens and the ceiling was increased to2 inches. The carbon monoxide was measured at 4.5 ppm and carbon dioxidewas 1.6%. The calculated oxygen free and water free carbon monoxidelevel was 38.7 ppm indicating an 81.5% destruction of carbon monoxide.

EXAMPLE 8

[0026] The same experiment was performed as in Example 6, except thatthe space between the catalyzed screens and the ceiling was increased to2 inches. The carbon monoxide was measured at 2.25 ppm and carbondioxide was 1.5%. The calculated oxygen free and water free carbonmonoxide level was 20.6 ppm indicating a 90.1% destruction of carbonmonoxide.

EXAMPLE 9

[0027] The same experiment was performed as in Example 8, except that a1.5 inch wide stainless steel foil was placed in front of and beneaththe catalyzed screens to force more of the exhaust through the catalyzedscreens. The carbon monoxide was measured at 1.6 ppm and carbon dioxidewas 1.5%. The calculated oxygen free and water free carbon monoxidelevel was 14.6 ppm indicating a 93% destruction of carbon monoxide.

EXAMPLE 10

[0028] The same experiment was performed as in Example 2, except that 2layers of 16 mesh, catalyzed screen was installed. The carbon monoxidewas measured at 6.3 ppm and carbon dioxide was 1.4%. The calculatedoxygen free and water free carbon monoxide level was 47.2 ppm indicatinga 78% destruction of carbon monoxide.

EXAMPLE 11

[0029] The same experiment was performed as in Example 10, except that 3layers of 16 mesh, catalyzed screen was installed. The carbon monoxidewas measured at 4.5 ppm and carbon dioxide was 1.4%. The calculatedoxygen free and water free carbon monoxide level was 19.2 ppm indicatinga 79% destruction of carbon monoxide.

EXAMPLE 12

[0030] The same experiment was performed as in Example 10, except that a1″ stainless steel curtain was installed to hang down at the forwardedge of the catalyzed screens. The carbon monoxide was measured at 5.8ppm and carbon dioxide was 1.5%. The calculated oxygen free and waterfree carbon monoxide level was 41.3 ppm indicating an 80% destruction ofcarbon monoxide.

EXAMPLE 13

[0031] The same experiment was performed as in Example 10, except that a1.5″ stainless steel curtain was installed to hang down at the forwardedge of the catalyzed screens. The carbon monoxide was measured at 5 ppmand carbon dioxide was 1.5%. The calculated oxygen free and water freecarbon monoxide level was 33.9 ppm indicating an 84% destruction ofcarbon monoxide. This shows the improvement with use of a curtain.

[0032] The results of these examples are summarized in Table 2 belowTABLE 2 Summary of Data from Examples EX. Mesh Size # of Screens Air DamCO Destruction, % 1 CONTROL  0 2 30 1 No 62 3 30 2 No 90 4 30 2 Yes 95 518 2 No   82.9 6 18 3 No   88.3 7 18 2 No 81.5 (2″ vs 1.5″ space) 8 18 3No 90.1 (2″ vs 1.5″ space) 9 18 3 Yes (1.5″)   93 (2″ vs 1.5″ space) 10 16 2 No 78 11  16 3 No 79 12  16 3 Yes (1)    80 13  16 3 Yes (1.5″) 84

The invention claimed is:
 1. A system to provide for oxidation of carbonmonoxide produced from gas log sets used in a vent free mode in a woodburning fireplace or fireplace insert comprising a vent chamberpositioned at the top of a combustion chamber, said vent chamber havingthree sides, a top, a floor having an opening therein for gaseouscommunication with said combustion chamber, an open front to ventoutside of the combustion chamber having a mouth, a catalytic structureis positioned in the opening in a frame so that a portion of exhaustfrom burning within the combustion chamber passes through the catalyticstructure into the vent chamber.
 2. The system according claim 1 whereinthe catalytic structure comprises at least one screen.
 3. The systemaccording to claim 2 wherein the screen has more than 30% open area. 4.The system according to claim 3 wherein the screen has more than 6meshes per square inch to less than 600 meshes per square inch and openarea in the range of 35 to 75%.
 5. The system according to claim 2wherein catalytic structure comprises more than one screen.
 6. Thesystem according to claim 2 wherein said screen is coated with a highsurface area ceramic chosen from aluminum oxide, cerium oxide, titaniumoxide, zirconium oxide or mixtures thereof and a catalytic component ofplatinum, palladium, cobalt or mixtures thereof.
 7. The system accordingto claim 2 wherein said screen is coated with a catalytic coating iscomposed of a high surface area oxide coating and a noble metalcatalytic component.
 8. The system according to claim 7 wherein thenoble metal comprises Pt. 9 The system according to claim 1 wherein saidcurtain extends below said frame across said mouth.
 10. The systemaccording to claim 9 wherein said curtain is porous.
 11. The systemaccording to claim 9 wherein said curtain is non porous.
 12. The systemaccording to claim 10 or 11 wherein said curtain is metallic.